ERBB2 and breast cancer: In recent years, several novel antibody-drug conjugates (ADCs) have been approved for the treatment of breast cancer, particularly human epidermal growth factor receptor 2 (HER2) positive metastatic breast cancer (MBC).1 Trastuzumab emtansine (T-DM1) was the first ADC to be approved in solid tumors, demonstrating a significant survival benefit in the EMILIA trial.2 At that time, T-DM1 was recommended as second-line treatment for HER2-positive MBC.